1. Field
Disclosed herein is a magnet core that is wound from a magnetically soft band. Also disclosed is to a method for producing such a magnet core and a fault current circuit breaker with a magnet core.
2. Description of Related Art
Magnet cores that are formed from a helically wound metal band, so-called ring band cores, are used in, for example, current transformers, power transformers, current-compensated radio interference suppression reactors, starting current limiters, storage reactors, single-conductor reactors, half-cycle transductors, and sum or difference current transformers for FI circuit breakers.
High demands are imposed on these cores with respect to magnetic properties: fault current transformers for AC-sensitive fault current circuit breakers, for example, must make available a secondary voltage that is at least enough to trigger the magnet system of the trigger relay that is responsible for shut-off. Since a design of a current transformer that saves as much space as possible is desired it is generally desirable that, a material for the magnet core high induction at the typical working frequency of 50 Hz, and also has a relative permeability μr that is as high as possible. The geometry of the magnet core and the material properties, in combination with the technological upgrading and processing of the material, for example by heat treatment, have a major influence on the relative permeability.
In the past, to achieve comparatively high relative permeabilities, it was necessary to achieve a saturation magnetostriction constant λs of |λs|<2 ppm, or even <0.3 ppm, and that was as small as possible. Moreover, bands that were as geometrically perfect as possible with as few defects of form as possible were an important prerequisite. However, it is only possible to easily achieve such a small saturation magnetostriction constant λs with only a few alloys. Moreover, for industrial production, it is almost impossible to achieve a sufficiently exact alloy composition without impurities.
It would, however, be possible to achieve high relative permeabilities with numerous other alloy compositions if the magnet core were free of mechanical stresses. Mechanical stresses are, for example, introduced into the magnet core when the core is wound from one or more bands or in its later handling or processing. The relationship between the absence of stresses in the magnet core and the high relative permeability is addressed in, for example, JP 63-115313. While stresses that have formed during winding can generally be greatly reduced in subsequent heat treatment, the delivery of mechanical stresses by external effects such as impacts or shaking must be avoided as much as possible.
For this purpose, for example, EP 0 509 936 B1 discloses connecting a magnet core made of a nickel iron alloy to a housing by means of a soft-elastic silicone cement by several bonding points. This process cannot, however, be transferred to a magnet core made of a magnetostrictive alloy since before complete crosslinking of silicone cement, it creeps as a result of capillary forces and the inherent weight of the magnet core between the band layers of the magnet core. Defects of form in amorphous and nano crystalline bands promote penetration of the cement. Upon curing, tensile stresses result on the crosslinked band layers, and thus the magnetic properties of the core are degraded. Since the intensity of penetration of the silicone cement between the band layers depends largely on randomly occurring defects of form, this effect can, moreover, only be predicted with difficulty and leads to serious variance of the permeability values.